The present invention relates generally to a nacelle for housing an aircraft engine and, more particularly, is concerned with a hybrid laminar flow nacelle effective for producing low friction drag, laminar flow at cruise operation and separation-free flow at off-cruise (takeoff or low speed) operation of an aircraft.
2. Description of the Prior Art
In a subsonic aircraft having an externally mounted engine, for example, a gas turbine engine mounted below a wing by a pylon, aerodynamic drag due to freestream airflow over the nacelle of the engine can typically represent approximately 4%of the total engine thrust output Any reduction in this aerodynamic drag can result in a significant saving in the amount of fuel consumed Thus, a desired function of an engine nacelle is to provide a lightweight housing for the aircraft engine which produces relatively low aerodynamic drag.
The aerodynamic drag due to a nacelle is determined by the pressure distribution and a dimensionless friction coefficient C.sub.f over the outer surface of the nacelle over which the freestream air flows during aircraft flight Reduced aerodynamic drag exists where the surface pressure distribution promotes a laminar boundary layer over the nacelle outer surface without any boundary layer separation thereof The friction coefficient C.sub.f, and thus aerodynamic drag, have reduced values when a laminar boundary layer exists Where the boundary layer along the nacelle outer surface transitions from laminar to turbulent, the friction coefficient C.sub.f, and thus aerodynamic drag, have increased values. Accordingly, it is desirable to provide a nacelle which promotes a surface pressure distribution effective for increasing the extent of laminar boundary layer flow, reducing the extent of turbulent flow and avoiding boundary layer separation.
Previous experience has demonstrated that a properly designed geometry of the outer surface of the nacelle can provide a favorable pressure gradient over an extended region of the nacelle, thus delaying the transition from laminar to turbulent flow. The result is a nacelle design with a lower friction or aerodynamic drag and a consequent reduction in fuel burn of 1.0 to 1.5% during cruise operation An example of such nacelle design is the natural laminar flow nacelle (NLFN) disclosed in D. J. Lahti et al U.S. Pat. No. 4,799,633, and assigned to the assignee of the present invention. The NLFN can result in a reduction of aerodynamic drag at cruise operation of the aircraft of approximately 50% when compared to prior art nacelles.
However, the NLFN with its emphasis on cruise performance has a relatively sharp-lipped leading edge (as compared to a blunt-lipped leading edge of a conventional nacelle) that is inadequate for off-cruise (takeoff or low speed, high angle-of-attack) operation of the aircraft Furthermore, during cruise operation of the aircraft, the NLFN may incur incipient spillage drag and wave drag sooner than the conventional nacelle (that is, at higher mass flow ratios and lower freestream Mach number respectively).
One conventional solution proposed for improving low speed operation of the NLFN by maintaining and extending laminar flow is variable geometry or leading edge systems such as flaps or translating slats, as recognized in the above-cited patent (see column 8, lines 49-55) While these appear to be viable solutions, the weight and mechanical complexity of such systems may cancel the benefits of the cruise drag reduction attributed to laminar flow produced by the NLFN design. In addition, these solutions require careful manufacturing to avoid steps and/or gaps in the external contour of the NLFN, when the system is retracted for high speed operation, that could result in premature transition to turbulent flow independent of the pressure gradient or distribution.
Another conventional solution proposed for maintaining and extending laminar flow on wings and nacelles has involved the use of active control devices, as also recognized in the above-cited patent (see column 2, lines 9-25). An active control device requires an auxiliary source of energy to cooperate with the surface for energizing or removing the boundary layer for maintaining laminar flow and preventing boundary layer separation. For example, boundary layer suction or blowing slots or holes disposed in the surface to be controlled are known in the art. The slot is connected to a pump by internal ducting and is effective for reducing or preventing turbulent flow, and thereby maintaining laminar boundary layer flow. Further, boundary layer bleed has been demonstrated successfully in maintaining laminar flow on airfoils (see NASA Contractor Report 165930 dated October 1982 entitled "Hybrid Laminar Flow Control Study-Final Report"). Also, boundary layer bleed has been demonstrated theoretically to be successful in maintaining attached flow on inlet lips at low speed, high angle-of-attack conditions (see AIAA-84-1399 dated June 1984 entitled "Analytical Study of Suction Boundary Layer Control for Subsonic V/Stol Inlets") However, the additional weight and energy required to power active control devices typically offsets advantages derived from the reduced aerodynamic drag.
For high speed operation, the NLFN is designed to a specific operating point, or mass flow ratio (MFR), to provide the favorable pressure gradient necessary to delay transition to turbulent flow. Decreasing the MFR below the design value can lead initially to premature transition to turbulent flow, thus losing the laminar flow drag advantage, and eventually to earlier spillage drag than a conventional nacelle. Also, since a relatively high Mach number near the maximum nacelle diameter is required to keep the boundary layer laminar, wave drag will become a problem at a lower freestream Mach number than for a conventional nacelle.
Despite the significant advantages and attainments attributed to the NLFN, it still represents less than an optimal design for producing low friction drag, laminar flow at cruise and separation-free flow at off-cruise aircraft operation. However, the conventional solutions referred to above do not unequivocally suggest which way one skilled in the art should proceed toward achievement of a more optimal design. Consequently, a need still remains for an alternative nacelle design more nearly approaching optimum performance.